Tuning Sodium Plating to Achieve Fast-Charging Anode-Free Sodium Batteries

Anode-free sodium batteries (AFSBs) have received significant research interest due to their high theoretical energy density, low cost, and enhanced safety. However, current AFSBs face challenges such as poor cycling stability and low rate performance, primarily because the efficiency of sodium (Na) plating and stripping on the current collectors is insufficient. Improving the electrochemical performance of AFSBs critically depends on optimizing Na plating and stripping behaviors. In our study, we discovered that the mechanical properties of the separators used in AFSBs play a crucial role in regulating Na plating. When separators exhibit in-plane isotropic mechanical properties, the Na metal deposits on the current collector with a flat and dense morphology, resulting in efficient Na plating and stripping. In contrast, if the separator's mechanical properties are anisotropic, Na dendrites tend to form and penetrate the separators, causing short circuits in AFSBs. Using optimally designed separators, AFSBs with a high mass loading of active materials (13.5 mg/cm²) demonstrated stable cycling performance at a current rate of 4 C. Beyond separators, we also explored the impact of the solvation structure of electrolytes on Na plating in AFSBs. We found that changes in the solvation structure altered the nucleation mode of Na metal on current collectors, which was crucial for achieving uniform Na plating. By optimizing the solvation structure, the critical current density of AFSBs was significantly enhanced, leading to improved fast charging performance. Our research provides valuable guidance for tuning Na plating and stripping behaviors to achieve high-rate and long-cycle-life AFSBs.